Curing of ink by radiation, and in particular by ultraviolet (UV) radiation, has become popular. UV curable inks often include monomers and photoinitiators. These monomers are often those capable of free radical polymerization. The growing end of each polymer chain is a radical that reacts with additional monomers, transferring the radical to the end of the chain as each monomer is added. The photoinitiator is used to form the first radicals to begin the polymerization process. As an example, the photoinitiator is capable of absorbing UV light to generate radicals to react with the monomers.
Two types of photoinitiators can be used in UV curable compositions. Type I photoinitiators are unimolecular photoinitiators that undergo a hemolytic bond cleavage upon absorption of UV light, forming radicals. Type-II photoinitiators are bimolecular photoinitiators. Type-II photoinitiators are a system that includes a photoinitiator with a co-initiator (such as a synergist or sensitizer), which together can form radicals upon exposure to UV light. With type-II photoinitiators, radicals are generated in a bimolecular process that involves the reduction of the photoexcited compound(s) by hydrogen abstraction, by energy transfer, or by electron transfer.